LIGHT AND VISION 



151 



is reflected, it is important to observe that it always leaves 

 the reflecting surface at the same angle as that at which it 

 falls upon it. If we wish to see ourselves in a mirror, we must 

 stand squarely in front of it. If we stand a little to one side, 

 we perceive only objects situated at the same angle to the 

 mirror on the opposite side. The scientific statement of the 

 fact is that "the angle of reflection equals the angle of inci- 

 dence." The principle is exactly illustrated in bouncing a 

 ball on the ground. If it strikes the earth at an angle, it 

 bounces up and away at the same angle. 



127. Refraction. A ray of light may be turned out of its 

 course in passing through a medium as well as when reflected 



A Y7 



FIG. 51, A prism changes the direction 

 of a ray of light. 



FIG. 52. Cross-sections of, 

 convex and concave lenses. 



from its surface. The turning occurs at the point where the 

 light passes from one medium into another of different density, 

 as from air to water, or from water to glass. If the light 

 strikes the surface of a transparent body at right angles, it 

 goes straight through, but if it strikes at any other angle it is 

 always sent out of its course in the direction of the denser 

 medium. The eye, however, is not cognizant of the changing 

 of the direction of the light rays, and sees the object from which 

 the light is reflected exactly as if the rays were direct. This 

 often results in curious illusions; in fact, the eye is probably 

 more easily deceived than any of our other senses. 



